Media for culturing bacteria are generally prepared by dispersing a solidifying agent in an aqueous solution containing nutrients and other ingredients necessary for the growth of specific microorganisms. Unfortunately, use of conventional solidifying agents is often inconvenient for the end-user. For example, when carrying out standard "plate count" or "pour plate" methods to determine the number of microorganisms in a liquid sample such as water or milk, the use of conventional agar medium is particularly inconvenient and time-consuming. The agar medium, which has generally been prepared in bulk and sterilized ahead of time, must be melted in boiling water or by exposure to flowing steam. The hot agar must then be carefully cooled to approximately 45.degree. C. prior to pouring into petri dishes. A series of dilutions of the test sample is then prepared and an aliquot of each dilution is placed in a petri dish. The cooled, but still liquified, agar medium is then poured into each dish, mixed with the aliquot of test sample, swirled to mix and allowed to solidify. After incubation, the number of colonies growing in each dish are counted by visual inspection. In this manner the number of microorganisms or colony-forming units present in the test sample can be determined.
It is apparent from the foregoing description that a simpler method of obtaining standard plate counts is desirable, particularly one that eliminates the need for the end-user to melt and cool the agar medium and pour it into the petri dishes.
The prior art has provided several gelling agents for microbiological growth media which are rehydratable at room temperature. For example, U.S. Pat. No. 3,046,201 suggests the use of certain polyacrylamides as gelling agents. U.S. Pat. No. 3,360,440 describes a microbiological medium in which the gelling agent is a cold-water-soluble modified cellulose. The aforementioned gelling agents are prepared by special processes involving expensive lyophilization procedures to increase the surface area of the dry powder to render it more easily rehydrated. When rehydrated, mixing is generally required to obtain a homogeneous gel.
U.S. Pat. No. 3,881,993 describes a device for assaying liquid specimens for microorganisms. One embodiment of the device comprises filter paper which is impregnated with a gelling agent and nutrients for growing microorganisms and which is adhered to a film by means of an adhesive layer. This embodiment suffers from the disadvantage that it is generally only semi-quantitative, due possibly to the presence of the filter paper. It is believed the filter paper is not suitably transparent and that it therefore renders counting of bacterial colonies difficult. Also, presence of the filter paper renders isolation of individual bacterial colonies impractical.